Discharge lamps, specifically HID (high-intensity discharge) lamps are used for a large area of applications where high light intensity is required. Especially in the automotive field, HID lamps are used as vehicle headlamps.
A discharge lamp comprises a sealed discharge vessel, which may be made e.g. from quartz glass, with an inner discharge space. Two electrodes project into the discharge space, arranged at a distance from each other, to ignite an arc therebetween. The discharge space has a filling comprising a rare gas and further ingredients such as metal halides.
An important aspect today is energy efficiency. The efficiency of a discharge lamp may be measured as lumen output in relation to the electrical power used. In discharge lamps used today for automotive front lighting an efficiency of about 90 lumen per Watt (lm/W) is achieved at a steady state operating power of 35 Watt.
During manufacture of known discharge lamps for automotive applications, it is conventional to use a bulb forming process to obtain a discharge vessel with at least externally ellipsoid shape.
U.S. Pat. No. 4,594,529 discloses a gas discharge lamp with an ionisable filling of rare gas, mercury and metal iodide. A lamp envelope is made of quartz glass and has an elongate discharge space, in which electrodes project. The discharge space of the lamp is circular-cylindrical. In a shown example, the inner diameter is 2.5 mm and the distance between the electrodes 4.5 mm. The lamp envelope has a comparatively thick wall to obtain a homogenous temperature distribution. The described lamp has a filling of Argon and 1 mg of Sodium Iodide, Scandium Iodide and Thorium Iodide in a molar ratio of 94.5:4.4:1.1 and obtains a luminous flux of 2500 lm in operation at a power of 35 W.